The present invention relates to a process, and to a device for carrying out said process, for making perforated films of regular and constant thickness at all points of the film. The invention also relates to the film thus obtained and to the finished products fashioned from this film.
Various processes starting from the extrusion of a film of synthetic material are known, in which perforations are made intermittently by the positioning of teeth which momentarily obturate the extrusion die.
In this way, films are obtained whose surfaces present appropriately distributed gaps or voids and in particular films comprising solid zones disposed longitudinally or transversely, alternating with these perforated zones formed by strands of matter separated by said gaps or voids.
The major drawback observed in the production of such films is the irregular thickness of the film thus produced, particularly at the level of the gaps or voids in the surface.
In fact, it is observed that, in the known devices, at the instant when the flow of matter is interrupted, then re-established, in order to constitute the apertures and gaps in the wall of the film, irregularities in thickness occur, the matter tending to form an excess thickness at the level of the edges defining the gaps and in particular at the intersection of the strands.
This particular feature presents various drawbacks:
Firstly, when the films thus formed are packed, for example wound on a reel, the excess thicknesses which come into register with one another provoke irregularities in the arrangement of the winding and it is difficult to obtain a regular winding over a great length.
In addition, when the film supplies a machine, either for continuous printing (on the solid zones) or to shape the film to produce finished products, for example bags for packaging, the surface irregularities lead to an irregular and sometimes unforeseeable path of the film in the machine, consequently causing defective shaping.
In particular, when the film, on leaving the die, is subjected to drawing in order to increase its dimensions and to cause molecular orientation at the level of the strands, intended to reinforce the mechanical strength thereof, the efforts of traction being transmitted irregularly due to the unequal thicknesses cause an irregular distribution of the matter; it is observed that the interlaced strands considerably resist drawing at the level of the nodes formed by the intersection of two strands (generally in excess thickness), whilst drawing principally affects the median zones of the strands located between two node; this results in an irregular drawing which is unaesthetic, and especially an unequal distribution of the efforts with the constitution of zones of weakness.
Finally, when the film is shaped to constitute a finished product such as a bag for packaging, the unequal thickness distributed in particular in the perforated zones produce a rough surface which is aggressive for the packaged products.
For all these reasons, it is desirable to obtain a continuously produced film presenting a partially perforated surface, without excess thickness especially at the level of the points of intersection of the strands defining the perforated zones.
It is an object of the invention to solve this problem and to produce a film which presents solid zones and perforated zones, the thickness of the solid zones and the perforated zones (constituted by the intersecting strands) being regular and constant.
Under these conditions, the film thus produced, without excess thickness, may be immediately subjected to a drawing operation causing the tubular film to be extended, with regular elongation of the strands composing the perforated zones.
A film is thus obtained which, after drawing, presents remarkable, considerably improved properties of mechanical strength whilst conserving the advantageous regular appearance of the film both in its perforated zones and in the solid zones.
Under these conditions, the film, after drawing, which presents the desired improved mechanical properties, may be subjected to the usual operations of winding on a reel, the absence of excess thickness allowing a compact, regular winding.
Subsequent unwinding of the film in treatment machines, for example for printing thereon, particularly in the solid zones, or for fashioning on automatic shaping machines, is facilitated, thanks to the regular unwinding of the film through the various operational phases.
The finished articles made with the film according to the invention, for example bags for packaging, etc . . . , may advantageously be used for containing fragile products such as vegetables without contact with the walls of the bag causing a slow deterioration of the surface or appearance of the products contained therein.
To this end, the invention relates to a process for making a tubular structure formed by a film of extruded synthetic matter whose walls are at least partially perforated, characterized in that a tubular film is extruded from an annular die, said film comprising gaps or voids obtained in manner known per se by teeth which temporarily obturate the outlet of said die, and in that the film is extruded and formed on leaving the die after flow of matter in the plastic state upstream of the die in a ring-shaped passage of which the outlet constitutes said die, this passage being of constant thickness equal to the width of the extrusion slot and consequently to the final thickness of the film, the flow of plastic material in this ring-shaped passage causing an effect of lamination of the matter ensuring dimensional stability, particularly the thickness of the film obtained at the outlet of the die, and in particular avoiding the formation of excess thickness or surface roughness on the film thus extruded.
The invention also relates to a device for carrying out the process and comprising an extrusion device of type known per se for forming a tubular film and the device is characterized in that the extrusion slot or die associated with a comb element animated by a reciprocating movement, in manner known per se, is preceded by a ring-shaped passage of thickness identical to the height of the extrusion slot and adapted to regulate the thickness of the extruded film of which the matter is subjected, during flow in said ring-shaped passage, to an effect of lamination, the film resulting from extrusion on leaving the die presenting a regular, constant thickness allowing in particular the passage of the film in a printing machine or in a shaping machine.